1. Field of the Invention
This invention relates to techniques having increased speed and accuracy for determining the amount of water in a sample via a Karl Fischer reaction, wherein each water determination does not require titration from one endpoint to another identical endpoint. More particularly, each water determination is made using a calibration of a sensor electrical current relative to the water equivalent of a known amount of titrant, together with the change in sensor electrical current that occurs when the sample is added to the K.F. reagent.
2. Description of the Prior Art
The determination of water content of a sample is important in many commercial products. For example, minute quantities of water in chemical process streams are detrimental for certain reactions. Further, the electrical properties of insulators are strongly dependent on water traces, and the water content of fluids such as gasoline has to be kept below a certain level. From these few examples, it is apparent that water determinations are among the most frequently performed analyses in many laboratories.
The currently most widely practiced water determination method is the "Karl Fischer" method, named after its originator who described the basis of this method in Zeitschrift Fuer Angewandte Chemie, Vol. 48, pages 394-396 (1935). In this method, the sample containing an unknown amount of water is added to a Karl Fischer reagent, hereinafter denoted K.F. reagent. This reagent is usually a solution of iodine and sulfur dioxide in pyridine and methanol or other solvents. Pyridine-free solutions are also well known in the art. The present state of the art is described in a book by E. Scholz entitled "Karl Fischer Titration", published by Springer-Verlag, N.Y. 1984. Typically, a titrant contains iodine or a mixture of iodine, SO.sub.2, and amine. The vessel solution (solution) is contained in a titration vessel and typically is an alcohol or a mixture of alcohol, SO.sub.2, and amine.
Present volumetric Karl Fischer titrators pretitrate to an endpoint, where an endpoint is defined as a small excess of a concentration of iodine in the vessel solution. After this, a sample is added to the titration vessel. The titration vessel is then titrated by adding an iodine-containing titrant so that the same endpoint is reached. The amount of water in the sample is determined based on the volume of Karl Fischer titrant that has been added to the sample-reagent solution in order to return to the same endpoint. To improve accuracy, these automatic titrators may correct for drift, which is usually due to water which enters the titrating vessel over time, even when no sample is added to the titration vessel. As is known in the art, drift can also be caused by many factors including the consumption of iodine by materials other than water which may be present in the titration vessel. However, the primary cause of drift generally is the water which leaks into the titration vessel from the atmosphere.
In these automatic titrators, the presence of iodine in the titration vessel is noted by a sensor. These sensors generally include two metal electrodes coupled to a sensing circuit selected from many well-known types. One commonly used circuit passes a constant current through the electrodes and measures the voltage across the electrodes. Another applies a constant voltage (d.c. or a.c.) to the electrodes and measures the current. When the amount of iodine in the solution is increased, the current level increases while, when a sample is added to the titration vessel, the amount of iodine is depleted and the current level decreases.
As noted, conventional titrators titrate from one endpoint to another identical endpoint. When the sample is present, the iodine concentration in the titration vessel decreases and the sensor current drops to a different level. After a period of a few seconds to allow the sample-reagent reaction to occur, iodine-containing titrant is added to increase the sensor current to its original level just prior to adding the sample. However, this is not always easy to accomplish, since titrant must be added very slowly as the desired current level is approached in order not to have an overshoot. The presence of any overshoot would cause error. Further, since the titrant is added very slowly toward the end of the titration step, such determinations require considerable amounts of time. In order to improve accuracy and speed in these conventional automatic titrators, two set points for the addition of titrants are frequently used. If the sensor current falls below the lower of the two set points, the titrator adds reagent at high speed. The next set point corresponds to the desired endpoint which is the current level just prior to when the sample was added to the titration vessel. Smaller amounts of titrant are added until this is reached. Other advanced titrators gradually reduce the titration speed as they approach the endpoint.
As noted, many variations of automatic titrators are found in the art, and are illustrated by the instruments described in U.S. Pat. Nos. 4,211,614 and 3,726,778. Further reference is made to the aforementioned text by E. Scholtz entitled "Karl Fischer Titration". These references generally describe various aspects of automatic titrators including endpoint drift correction.
As is apparent from the foregoing, automatic titration methods and apparatus in which titration must be from one endpoint to another identical endpoint require a considerable amount of time for each sample water determination, and are prone to errors due to overshoots in adding titrant. Each water determination requires that titrant be added and, whether or not multiple set points are used to approach the desired endpoint, a considerable amount of time is needed for each water determination. Moreover, the accuracy and sensitivity of each titration is limited by the accuracy of the mechanical components, i.e., the buret drive, of the titrator. The smallest amount of water that can be measured is determined by the precision of that buret drive. In contrast with this, the present invention does not require titration from one endpoint to another identical endpoint. In some cases it does not even require the addition of titrant in order to determine the water content of a sample, i.e., water amounts smaller than those given by the mechanical precision of the titrator can be measured. Still further, drift corrections can be made as the water determinations proceed, the method and apparatus providing improved accuracy and speed over conventionally known titrators.
Accordingly, it is a primary object of the present invention to provide titration techniques that exhibit improved speed and accuracy when used for water determination by the Karl Fischer method.
It is another object of this invention to provide improved automatic titration methods wherein water sample determinations can be made without necessarily requiring the addition of a titrant to a vessel solution containing the K.F. reagent and the sample to be analyzed.
It is another object of this invention to provide improved automatic titration methods wherein continuously updated drift corrections can be applied.
It is another object of this invention to provide improved automatic titration methods for determining water content of a sample by Karl Fischer reactions wherein the necessity of titrating from one endpoint to another identical endpoint is not required.
It is another object of this invention to provide improved automatic titration methods wherein continuous drift correction can be implemented in a very small amount of time, leading to improved accuracy of water determination.
It is another object of this invention to provide a technique that will quickly enable the validation of previously obtained results to determine water content.